The generation, detection, and use of THz radiation (that is radiation having frequencies in the range 50 GHz-20 THz) is known. One application utilising the generation and detection of THz radiation is THz time-domain spectroscopy. This is a spectroscopic technique in which short pulses of THz radiation are generated and used to probe material properties. The generated THz pulses are directed towards a sample of the material or the object to be probed, and the radiation subsequently transmitted through the sample or object, or reflected from the sample or object is detected. The use of THz radiation has several advantages over other forms of spectroscopy. These include the fact that many materials are transparent to THz radiation, THz radiation is safe for biological tissues, and images formed with THz radiation can have good resolution (for example less than 1 mm). Also, many potentially interesting materials have unique spectral fingerprints in the THz range, which means that THz radiation can be used to identify them. These materials include certain types of explosives, and compounds used in commercial medicines and certain illegal substances. As many materials are transparent to THz radiation, the items of interest can be observed through visually opaque intervening layers, such as packaging and clothing.
Typically, THz pulses are generated by a short pulsed laser, and last only a few picoseconds. Known techniques for generating THz pulses include surface emission from a semiconductor surface illuminated by an ultra-short optical pulse, and emission from a voltage biased photoconductive emitter; in both cases the optical laser pulse creates electron-hole pairs in a semiconductor material which may be accelerated to generate THz radiation. Another technique is optical rectification, in which short laser pulses pass through a transparent crystal material which then emits a THz pulse without any applied voltages.
A variety of techniques are also known for the detection of THz pulses, including photoconductive detection, in which an electrical current is produced between a pair of electrodes (or antenna leads). This current is generated by the THz electric field pulses acting on electrons/holes pairs which have been themselves been excited by a short laser pulse.focused onto a semiconductor surface. After the THz electric field generates a current across the antenna leads, this may then be amplified using a suitably arranged amplifier. Another method of detecting THz pulses is electro-optical sampling.
While terahertz time-domain spectroscopy (THz-TDS) is a widely-used technique applicable to the study of many systems, and its usefulness as a non-destructive tool for spectroscopy, imaging, monitoring, and detection of materials has been demonstrated across many application areas, laboratory-based systems are typically large, cumbersome and expensive. This is owing primarily to the Ti:sapphire laser (centre wavelength 800 nm) used to provide pulsed excitation with sufficient energy to exceed the band-gap of the most common semiconductor system used for THz emission, gallium arsenide. THz emission from alternative materials with smaller band-gaps than gallium arsenide could in principle provide a compelling solution to these problems, allowing more portable THz spectroscopy systems to be made.
Materials with potentially suitable, smaller band-gaps include InAs, InGaAs, and InAlAs. However, a problem in trying to use these materials for the generation of THz radiation is that, when grown, these materials tend to have low resistivities. As will be appreciated, in typical techniques for generating THz radiation, relatively large bias voltages or potentials (some tens of volts) are applied between electrodes, to accelerate the electrons and holes of the electron-hole pairs excited in the semiconductor material by incident illuminating radiation with photon energies greater than the relevant band-gap. If the resistivities of the semiconductor materials are low, then the application of these bias voltages (or equivalently these bias electric fields) drives unacceptably large currents between the electrodes, through the semiconductor material, causing it to be damaged.
As a result of these problems, there remains the need for methods and apparatus for THz emission from materials excited by telecoms wavelengths, which would allow comparatively cheap 1.55 micron wavelength lasers to replace the Ti:sapphire laser, for example.